1. Field of the Invention
The present invention relates to a deterioration determination apparatus for an exhaust emission control device used for internal combustion engine and, in particular, relates to a deterioration determination apparatus for an exhaust emission control device comprising catalysts such as a so-called three way catalyst and an HC adsorbing catalyst.
2. Description of the Related Art
There is conventionally known an HC adsorbent characterized by adsorbing unburned hydrocarbon (to be referred to as simply "HC") which is generated at low temperature, for example, at the cold start of the internal combustion engine such as gasoline engine (to be referred to as simply "engine" hereinafter) and desorbing the adsorbed HC at high temperature.
As a deterioration determination apparatus for an exhaust emission control device comprising such an HC adsorbent, there is proposed an apparatus for an exhaust emission control device comprising an HC adsorbent in the upstream of a so-called three way catalyst provided in an exhaust passage, as disclosed in Japanese Patent Application Laid-open No. 6-66131.
More specifically, in the deterioration determination apparatus of this type, so-called O.sub.2 sensors for detecting whether the air-fuel ratio of exhaust gas is on the lean side or on the rich side with respect to a theoretical air-fuel ratio are provided in the upstream of the HC adsorbent, i.e., between the engine and the HC adsorbent, and in the downstream of the HC adsorbent, i.e., between the HC adsorbent and the three way catalyst, respectively.
In this deterioration determination apparatus, the so-called inversion frequency ratio of the outputs of both of the O.sub.2 sensors in the upstream and downstream is obtained and the deterioration determination of the adsorption capability of the HC adsorbent is made based on the inversion frequency ratio.
It is noted that, in this deterioration determination apparatus, the feedback controlling of the air-fuel ratio of mixture gas supplied to the engine, i.e., so-called air-fuel ratio feedback control is also conducted based on the output of the upstream O.sub.2 sensor.
There is also known an HC adsorbing catalyst characterized by adsorbing the HC and desorbing the adsorbed HC at high temperature as the HC adsorbent and also characterized by having a catalytic action of converting the desorbed HC into an innocuous substance.
Some of the exhaust emission control devices provided with an HC adsorbing catalyst as well as the three way catalyst have a structure in which an HC adsorbing catalyst is provided in the downstream of the three way catalyst provided in the exhaust passage in consideration that the HC adsorbing catalyst has a low degree of heat endurance ability, as disclosed in Japanese Patent Application Laid-Open No. 7-144119.
In the exhaust emission control device of this type, if the engine has just been cold and the three way catalyst is inert, the HC which cannot be converted into an innocuous substance by the inert three way catalyst is adsorbed first by the HC adsorbing catalyst.
After the three way catalyst is activated, the activated three way catalyst converts the HC into an innocuous substance. Thereafter, if the HC adsorbing catalyst is activated finally, then the activated HC adsorbing catalyst converts the HC adsorbed by the catalyst into an innocuous substance.
In the above-stated exhaust emission control device, the feedback controlling of the air-fuel ratio of the adsorbed mixture gas is conducted by using the O.sub.2 sensor provided in the upstream of the three way catalyst in accordance with timing at which the three way catalyst is activated.
Needless to say, with the exhaust emission control device provided even with the HC adsorbing catalyst of this type, if the HC adsorbing catalyst deteriorates, there is a possibility that the HC is not sufficiently adsorbed or, if adsorbed, the absorbed HC can not be converted into an innocuous substance to a sufficient degree. The need of a deterioration determination apparatus capable of determining whether the HC adsorbing catalyst deteriorates with high accuracy is high and demands for realizing such an apparatus are arising.
Furthermore, as for the above-stated exhaust emission control device, if the determination of the deterioration of not only the HC adsorbing catalyst but also of various catalysts such as a three way catalyst can be made, more accurate determination of the deterioration of the exhaust emission capability can be made. For that reason, demands are also arising for the realization of a deterioration determination apparatus capable of determining not only the HC adsorbing catalyst but also a three way catalyst and the like.